Suspecting bad ECU and master cylinder. Please help confirm. (pretty long)

Dan Bozga audisport44 at
Mon Nov 9 18:16:02 PST 2009

Hi everyone,

I'll start by saying that I've been part of this mailing list before, many years ago. I doubt anyone will remember though. I'm on my 4th 20V now having owned a red one at first followed up by a black and then panther black. I haven't had one for quite a few years but I recently acquired a pearl white in hopes of using as a daily driver so I can stop driving my '02 B5 S4 all the time. I've put way too much time and money into that car so I really need to start racking up miles on something else. That's where the 200 comes in.

I bought the car from a guy that seemed to be clueless about it or what it really was and he thought it had a bad turbo. This was according to his mechanic that works on BMWs, so you can imagine how familiar he is with Audis. The turbo itself is fine, not seized by any means and the shaft play is so minimal it's one of the best I've seen. There were many other little issues here and there which I've been fixing but the two main problems still remain.

1. The brakes. 

The brake light stays on constantly. At first everything is ok, but eventually after driving the car for a very short while they begin to seize up. The brake pedal actuation point drops down and the car wants to haul to a stop and you really have to use the engine to overpower the brakes and get it moving. I've checked all the calipers because at first I thought it was corroded carrier pins. All 4 calipers seem to have been replaced pretty recently. I took out all the pins front and rear and cleaned and greased them either way. I no longer think it's a matter of stuck carriers and somehow doubt it would be pistons either. The brake servo (part that connects to the firewall and brake pedal) was leaking pretty badly so I thought that might be why it seizes. I replaced it with a used but good unit with about half the miles of mine (198k on this car btw). The leak has stopped but the problem is still there. 

I've checked power assistance after the car is stopped and I have plenty of it. I've pumped the brakes like 40 times and there was still assistance. All of this leads me to believe that the pressure accumulator is fine and that the next thing I should be replacing is the master cylinder. Once you park the car the pedal actuation point gradually start to return and you can hear a "seized piston becoming loose" type of sound when this happens. Before I throw more parts at it I wanted to see what you guys think and if you agree with my direction here.

2. The engine.

It's a bit hard to start at first and wants to stall but once it gets going the idle is smooth and stays on no problem. I think the issue here might be fuel pressure decreasing after shut off. If you stop it, it starts right back up. This problem is only there after its been sitting for a while. I've checked the carbon canister solenoid and it doesn't seem to be getting power. I did the test for it by jumping the posts under the knee bolster and there was no clicking sound at all. I replaced it with another unit and that didn't solve it either. I measured resistance on both valves that I have and they check out. Bentley says there is supposed to be continuity between terminal 2 from this valve/solenoid connector and fuse #24 but there was none. I tried tracing wires but couldn't come up with much. I don't know exactly how they are routed, it gets pretty fuzzy once you get between the fuse box and under dash relay wires. Speaking of relays, there is an
 empty spot(knee bolster) with actual wires going to it labeled "IRA relay". Any of you know what it stands for and if it's supposed to be present? There are also two spots for relays listed on the fuse box cover for AC under spots "6" and "11". Are these also supposed to be present or are they empty spots? They both have wires going to them too. Haven't tested AC on this car yet. The seller said it doesn't work anyways so I assume bad compressor (looks like it leaks) but that hasn't been my priority right now.

When in neutral you can open the throttle all the way and rev the car up but it gets stuck around 4250 RPMs at which point it begins to sputter violently and pop VERY loudly. I think it's hitting the fuel cut because the dash displays 1.9 bar when this happens. That's the highest I was able to get it so far. I've never seen an Audi make full (or more) boost in neutral so this led me to believe the car is overboosting for some reason. When this violence takes place you can hear the ECU clicking constantly too. If you just let it idle after one of these episodes (aka hitting that 4250 spot for a few seconds) the wastegate frequency valve starts clicking as if it's receiving a bad signal. Now I've checked this valve with one out of my S4 and although they are not the same part numbers I figured it should still show a sign of improvement. Nothing changed. I've been down the road of checking every component with the output test and a multimiter. Everything
 seems to work fine. 

There were a couple boost leaks which I fixed and now the system can hold 20psi no problem. I took apart the wastegate suspecting a bad diaphragm but it is in perfect shape. No holes at all. I checked it very well twice. I know what a bad one looks like from my first Audi ('86 5K CS Q) and can assure you it isn't the problem. I feel like I've eliminated everything at this point which made me start looking at the ECU. I had a feeling it wasn't stock, even thought the rest of the car is all original. Well, what do you know? When I took it apart I noticed the higher value(2.5) BOSCH pressure transducer and aftermarket eproms (both of them). I took pictures of all of these so maybe you guys can help me figure out what company's software this is. The little white labels say HOB  on top outlined by black marker. The rest should be very legible. Also when you take these stickers off there is a little glass window which allows you to see inside the chip. Took a
 picture of that too hoping it might help identifying them.

The fault codes I get are:

2111 (rpm sensor). I checked out the flywheel and all the teeth are fine. So are the timing reference pins. This led me to believe it might be a faulty RPM sensor. I replaced it with a known good one and it made no difference.

2341 (oxygen sensor limit exceeded). The car is old and I don't know its history so I figured I'd replace the O2 sensor anyways with a genuine BOSCH OEM unit (no splicing). Of course this didn't solve the problem but I thought I'd put it out there. 

What else can I say to help...

I checked the distributor gear and it actually has the updated metal unit so no issue there either. I've done checks on everything from air temp sensor to TPS and everything in between. It all looks fine and/or measured within spec.

When the car is under load you can still only get it up to 4250 RPMs before all hell breaks loose. You have to get it up there gradually though. It WILL cut off at any point if you attempt to floor it (unlike in neutral). 

Pretty much what I've come up with so far is that a part of the ECU is faulty and causing mayhem. What would limit the amount of boost the car makes? The pressure transducer or one of the eproms? I'm thinking this is where the codes and overboost are coming from. I'd love to swap the ECU with a known working unit but I don't have that luxury. Maybe putting in stock chips and psi trans. might work. I don't know, I'm open for suggestions before I go out buying another computer. The board seems to be socketed but I don't have the stock chips or transducer to check.

I'm in the Washington D.C. are if any of you are local. Or, I could perhaps send my ECU to one of you guys to test it in your car. I'd offer payment for anyone's trouble.

Here's a link to the 5 pictures I took:

Please click on each one you'd like to see and then click again to view the full size.

Thanks for listening. I'm sorry it's so long, but I tried to make it as detailed as possible. If you need ANY other details just ask away.

Dan B.

P.S. I did a fresh full on timing belt kit on this car so there is no problem with any timing marks being misaligned. It had no impact on my issue but I had to do it anyways. Again, not knowing the history of the car.


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